In order to study the frequency scattering mechanism of capacitance in latticematched In _0.17Al _0.83N/GaN high electron mobility transistors (HEMTs), the latticematched In _0.17Al _0.83N/GaN heterojunction Schottky diodes with circular planar structure, which have equivalent capacitance characteristics to those of HEMTs, are fabricated and tested in this paper. The experimental curves of capacitance-voltage characteristics at different frequencies show that the capacitance of the accumulation area decreases gradually with the increase of frequency at low frequency, which accords with the capacitance frequency scattering characteristics of traditional HEMT devices. However, when the frequency is higher than 200 kHz, the capacitance of the accumulation area increases rapidly with frequency increasing, which cannot be explained by the traditional capacitance model. By comparing the reverse current and capacitance characteristics of latticematched In _0.17Al _0.83N/GaN Schottky diodes, it is observed that the saturation behavior of the reverse leakage current is clearly associated with full depletion of the two-dimensional electron gas at the InAlN/GaN interface, which is indicated by the rapid drop of the diode capacitance. This observation suggests that the large reverse leakage current of the lattice-matched In _0.17Al _0.83N/GaN Schottky diode, which reaches up to 10 ^-4A, should has a direct influence on the capacitance scattering. By considering the influence of leakage current, interface state and series resistance comprehensively, the capacitance frequency scattering model is modified based on the traditional model. Using various models to fit the experimental capacitance-frequency data, the results from the modified model agree well with the experimental results. According to the parameters obtained by fitting, the density and the time constant of interface defects in latticematched In _0.17Al _0.83N/GaN Schottky diodes, determined by equivalent interface capacitance and resistance, are about 1.66×10 ¹⁰cm ^-2·eV ^-1and 2.65μs, respectively. According to the values reported in the literature, it is suggested that the modified capacitance frequency scattering model should be reasonable for explaining the capacitance scattering phenomenon in accumulation area. In conclusion, we believe that the capacitance of latticematched In _0.17Al _0.83N/GaN Schottky diode scatters is a joint result of leakage current, interface state and series resistance. The interface defects in In _0.17Al _0.83N/GaN Schottky diodes usually have a great influence on frequency and power characteristics of devices, a correct explanation for the frequency scattering mechanism of capacitance is the basis for determining the locations and sources of defects in Ⅲ nitride devices.